A background technology for a substrate stacked image sensor will be described in two aspects. One is an aspect according to stacking of a semiconductor integrated circuit, and the other is miniaturization of an image sensor.
Hereinafter, a conventional technology for the stacking of the semiconductor integrated circuit will be described. As the semiconductor integrated circuit is continuously manufactured in a micro size, a packaging technology has also been continuously developed in order to satisfy a demand for miniaturization and mounting reliability. Recently, there have been developed various technologies for substrate stacking with a three-dimensional (3D) structure in which two or more semiconductor chips or semiconductor packages are vertically stacked.
An element with a three-dimensional (3D) structure using such substrate stacking is subject to substrate stacking, to a process (thinning) of grinding the rear surface of a stacked substrate in order to reduce the thickness after substrates are stacked, to a subsequent manufacturing process, to a sawing process, and to a packaging process.
An image sensor vertically stacked using such substrate stacking is manufactured by performing separate processes for a first substrate and a second substrate to complete element manufacturing, aligning the two substrates to be folded, and electrically and mechanically bonding the two substrates to each other.
Many conventional technologies of stacking substrates exist in various fields, and various technologies have been attempted by the present applicants. For example, a method, in which an image sensor can be more economically manufactured by omitting a process for bonding, stacking, and etching substrates, is disclosed in Korean Patent Application No. 2010-0015632 filed in Feb. 2, 2010 by the present applicant.
Furthermore, a technology for minimizing a misalignment problem of bonding pads on substrates when bonding the substrates is also disclosed in Korean Patent Application No. 2010-0046400 filed in May 18, 2010 by the present applicant.
Furthermore, a manufacturing method for allowing pads on substrates to protrude in order to facilitate bonding when bonding the substrates is also disclosed in Korean Patent Application No. 2010-53959 filed in Jun. 8, 2010 by the present applicant.
In the background technology according to the miniaturization of an image sensor, with the development of a mobile appliance such as a cellular phone, only when the height of a camera module embedded in the mobile appliance should be reduced and the resolution of an image sensor included in the camera module should be increased, the degree of design in the mobile appliance is increased. By such trend, a pixel size of the image sensor has also been continuously reduced.
Recently, with the development of a semiconductor integrated circuit technology, a pixel can be manufactured with a size of about 1.4 μm×about 1.4 μm approximating to a wavelength band of a visible ray. Therefore, in a conventional front side illumination (FSI) scheme, light incident from an exterior may not be sufficiently collected in photodiodes due to interference of metal lines. In order to solve such a problem, there has also emerged a back side illumination (BSI) image sensor in which photodiodes are arranged as near as possible to a direction in which light is incident.
FIG. 1 schematically illustrates such a BSI image sensor, and three-dimensionally illustrates four unit pixels including red, green, and blue color filters 11, 21, 31, and 41 and photodiodes 12, 22, 32, and 42, respectively. FIG. 2 illustrates only a red pixel among the pixels. It is noted that FIG. 1 to FIG. 3 illustrate only a color filter part and a photodiode part formed in a semiconductor substrate among the pixels constituting the image sensor pixels.
Furthermore, with the continuous development of a semiconductor technology, also in the BSI image sensor, the depth of the pixel reaches about 3 μm to about 5 μm and the width of the pixel is reduced to about 1.1 μm as illustrated in FIG. 2, so that many more pixels can be integrated per unit area. In this case, a signal disturbance phenomenon, which has not been serious in the conventional art, has been a new problem.
Such a problem will be described in more detail with reference to FIG. 3 that is a sectional view of two continuously arranged pixels.
In FIG. 3, light incident through a green color filter 21 generates photoelectrons in a corresponding photodiode 22. Most of the photoelectrons are normally captured in a depletion region (indicated by dotted lines in FIG. 3) of the photodiode 22 connected to the green color filter 21, and form a valid current component. However, a part of the photoelectrons move to a photodiode 12 of an adjacent pixel, wherein the number of the photoelectrons moving to the photodiode 12 increases as the widths of the photodiodes 12 and 22 are narrowed. This represents loss of a signal in the photodiode 22 connected to the green color filter 21, and serves as a unnecessary signal, that is, color noise in the photodiode 12 connected to the red color filter 11. This is called a crosstalk phenomenon. As a result, in a pixel with a depth of about 3 μm to about 5 μm and a narrow width of about 1.1 μm, since the crosstalk phenomenon becomes serious, the advantages of the BSI scheme disappear.
In the case in which the size (interval) of a pixel is about 1.1 μm, the thickness of a substrate is reduced to a thickness equal or less than a half (for example, a substrate thickness of 4 μm→a substrate thickness of 2 μm) thereof in order to reduce the crosstalk phenomenon, resulting in an increase in a ratio in which incident light is not sufficiently absorbed by a silicon photodiode and transmits the photodiode. That is, since quantum efficiency (QE) is reduced, the amplitude of an electrical signal is further reduced. The quantum efficiency (QE) indicates a ratio of charge generated/captured by incident light, that is, incident photons, with respect to the incident light, and is an index related to efficiency by which a light signal is effectively converted into an electrical signal by an image sensor.
Furthermore, in the conventional BSI image sensor, when a thickness is reduced in order to solve the crosstalk, it is well known that most of blue light is absorbed by a photodiode of a first substrate but green light is partially absorbed. Red light is also partially absorbed but the absorption amount is smaller than that of the green light. In addition, the absorption amount of infrared is very small as compared with them.
Since the absorption of light indicates that photons have been converted into charge, there is a problem that quantum efficiency (QE) is reduced in sequence of blue light>green light>red light>infrared. Furthermore, since a non-absorbed light component is absorbed in a part other than a photodiode, is scattered after colliding with metal lines, or deeply transmits a stacked substrate, and finally has no relation to the quantum efficiency, there is a problem that the waste of light occurs.